a

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The first letter of the modern English alphabet.
2. n. Any of the speech sounds represented by the letter a.
3. n. The first in a series.
4. n. Something shaped like the letter A.
5. n. The best or highest in quality or rank: grade A milk.
6. n. Music The sixth tone in the scale of C major or the first tone in the relative minor scale.
7. n. Music A key or scale in which A is the tonic.
8. n. Music A written or printed note representing this tone.
9. n. Music A string, key, or pipe tuned to the pitch of this tone.
10. n. One of the four major blood groups in the ABO system. Individuals with this blood group have the A antigen on the surface of their red blood cells, and the anti-B antibody in their blood serum.
11. idiom. from A to Z Completely; thoroughly.
12. Used before nouns and noun phrases that denote a single but unspecified person or thing: a region; a person.
13. Used before terms, such as few or many, that denote number, amount, quantity, or degree: only a few of the voters; a bit more rest; a little excited.
14. Used before a proper name to denote a type or a member of a class: the wisdom of a Socrates.
15. Used before a mass noun to indicate a single type or example: a dry wine.
16. The same: birds of a feather.
17. Any: not a drop to drink.
18. prep. In every; to each; per: once a month; one dollar a pound.
19. v. Informal Have: He'd a come if he could.
20. abbr. acceleration.
21. abbr. are (measurement)

Century Dictionary and Cyclopedia

1. The first letter in the English alphabet, as also generally in the other alphabets which, like the English, come ultimately from the Phenician. Our letters are the same as those used by the Romans; the Roman or Latin alphabet is one of several Italian alphabets derived from the Greek; and the Greek alphabet is, with a few adaptations and additions, formed from the Phenician. As to the origin of the Phenician alphabet, opinions are by no means agreed; but the view now most widely current is that put forth and supported a few years ago by the French scholar De Rougé: namely, that the Phenician characters are derived from early Egyptian hieratic characters, or abbreviated forms of written hieroglyphs. Under each letter will be given in this work the Phenician character from which it comes, along with an early form or two of the Greek and Latin derived characters (especially intended to show the change of direction of the letter consequent upon the change of direction of writing, since the Phenician was always written from right to left); and to these will be added the hieratic and hieroglyphic characters from which the Phenician is held to originate, according to De Rougé's theory. It is to be noticed that our ordinary capitals are the original forms of our letters; the lowercase, Italic, and written letters are all derived from the capitals. Our A corresponds to the Phenician letter called aleph; and this name, signifying “ox,” is also the original of the Greek name of the same letter, alpha. The comparative scheme for A is as follows: The Phenician aleph was not a proper vowel-sign, but rather a quasi-consonantal one, to which an initial vowel-sound, of whatever kind, attached itself; since the fundamental plan of that alphabet assumed that every syllable should begin with a consonant. But the Greeks, in adapting the borrowed alphabet to their own use, made the sign represent a single vowel-sound: that, namely, which we usually call the “Italian” or “Continental” a (ä), as heard in far, father. This was its value in the Latin also, and in the various alphabets founded on the Latin, including that of our own ancestors, the speakers and writers of earliest English or Anglo-Saxon; and it is mainly retained to the present time in the languages of continental Europe. In consequence, however, of the gradual and pervading change of utterance of English words, without corresponding change in the mode of writing them, it has come to have in our use a variety of values. The sound of a in far is the purest and most fundamental of vowel-sounds, being that which is naturally sent forth by the human organs of utterance when the mouth and throat are widely opened, and the tone from the larynx suffered to come out with least modifying interference by the parts of the mouth. On the other hand, in the production of the i-sound of machine or pique and the u-sound of rule (or double o of pool), the organs are brought quite nearly together: in the case of i, the flat of the tongue and the roof of the mouth; in the case of u, the rounded lips. Hence these vowels approach a consonantal character, and pass with little or no alteration into y and w respectively. Then e and o (as in they and note) are intermediate respectively between a (ä) and i and a (ä) and u; and the sounds in fat and fall are still less removed in either direction from a (ä). The pure or original sound of a (far) is more prevalent in earlier stages of language, and is constantly being weakened or closened into the other vowel-sounds, which are to a great extent derived from it; and this process has gone on in English on a larger scale than in almost any other known language. Hence the a-sound (as in far) is very rare with us (less than half of one per cent. of our whole utterance, or not a tenth part as frequent as the sound of i in pit or as that of u in but); its short sound has been so generally flattened into that in fat, and its long sound into that in fate, that we now call these sounds respectively “short a” and “long a”; and, on the other hand, it has in many words been broadened or rounded into the sound heard in all and fall. Thus the most usual sounds of English written a are now, in the order of their frequency, those in fat, fate, fall, far; there are also a few cases like the a in what and was (after a w-sound, nearly a corresponding short to the a of all), many (a “short e”), and others yet more sporadic. In syllables of least stress and distinctness, too, as in the first and third syllables of abundant and abundance, it is universally uttered with the “short u” sound of but. The “long a” of fate is not strictly one sound, but ends with a vanishing sound of “long e”: i. e., it is a slide from the e-sound of they down to the i-sound of pique. From this vanish the a of fare and bare and their like is free, while it has also an opener sound, and is even, in the mouths of many speakers, indistinguishable in quality from the “short a” of fat; hence the a-sound of fare is in the respellings of this work written with ã, to distinguish it from the sound in fate. There is also a class of words, like ask, fast, ant, in which some pronounce the vowel simply as “short a,” while some give it the full open sound of a in far, and yet others make it something intermediate between the two: such an a is represented in this work by ȧ. A occurs as final only in a very few proper English words; and it is never doubled in such words.
2. As a symbol, a denotes the first of an actual or possible series. Specifically
3. In music, the name of the sixth note of the natural diatonic scale of C, or the first note of the relative minor scale; the la of Italian, French, and Spanish musicians. It is the note sounded by the open second string of the violin, and to it as given by a fixed-toned instrument (as the oboe or organ) all the instruments of an orchestra are tuned.
4. In the mnemonic words of logic, the universal affirmative proposition, as, all men are mortal. Similarly, I stands for the particular affirmative, as, some men are mortal; E for the universal negative, as, no men are mortal; O for the particular negative, as, some men are not mortal. The use of these symbols dates from the thirteenth century; they appear to be arbitrary applications of the vowels a, e, i, o, but are usually supposed to have been taken from the Latin AffIrmo, I affirm, and nEgO, I deny. But some authorities maintain that their use in Greek is much older.
5. In mathematics: In algebra, a, b, c, etc., the first letters of the alphabet, stand for known quantities, while x, y, z, the last letters, stand for unknown quantities; in geometry, A, B, C, etc., are used to name points, lines, and figures.
6. In abstract reasoning, suppositions, etc., A, B, C, etc., denote each a particular person or thing in relation to the others of a series or group.
7. In writing and printing, a, b, c, etc., are used instead of or in addition to the Arabic figures in marking paragraphs or other divisions, or in making references.
8. In naut. lang., A1, A2, etc., are symbols used in the Record of American and Foreign Shipping, and in Lloyd's Register of British and Foreign Shipping, to denote the relative rating of merchant vessels. In the former, the character assigned to vessels by the surveyors is expressed by the numbers from 1 to 3, A1 standing for the highest and A3 for the lowest grade. The numbers 1½, 1¾, 2, 2½ express intermediate degrees of seaworthiness. Vessels classed as A1 or A1½ are regarded as fit for the carriage of all kinds of cargoes on all kinds of voyages for a specified term of years; those classed as A1¾ or A2, for all cargoes on Atlantic voyages, and in exceptional cases on long voyages, and for such cargoes as oil, sugar, molasses, etc., on any voyage; those classed as A2½ or A3, for coasting voyages only, with wood or coal. In Lloyd's Register, the letters A, A (in red), Æ, and E are used to denote various degrees of excellence in the hulls of ships, the figure 1 being added to express excellence of equipment, such as masts and rigging in sailing-ships, or boilers and engines in steamers. The broad A in the British Lloyd's indicates a ship built of iron. In the American Register, the annexed figures do not refer to the equipment.— Hence, in commerce, A1 is used to denote the highest mercantile credit; and colloquially A1, or in the United States A No. 1, is an adjective of commendation, like first-class, first-rate: as, an A1 speaker.
9. As an abbreviation, a stands, according to context, for acre, acting, adjective, answer, are (in the metric system), argent (in heraldry), anal (anal fin, in ichthyology), anechinoplacid (in echinoderms), etc.; in com., for approved, for accepted, and for Latin ad (commonly written @), “at” or “to”: as, 500 shares L. I. preferred @ 67½; 25 @ 30 cents per yard.
10. Attrib., having the form of the capital A, as a tent.
11. The form of an used before consonants and words beginning with a consonant-sound: as, a man, a woman, a year, a union, a eulogy, a oneness, a hope. An, however, was formerly often used before the sounds of h and initial long u and eu even in accented syllables (as, an hospital, an union), and is still retained by some before those sounds in unaccented syllables (as, an historian, an united whole, an euphonious sound). The form a first appeared about the beginning of the thirteenth century. It is placed before nouns of the singular number, and also before plural nouns when few or great many is interposed. [Few was originally singular as well as plural, and the article was singular (ME. a) or plural (ME. ane) to agree with it. In the phrase a great many, the article agrees with many, which is properly a noun (AS. menigu: see many, n.); the following plural noun, as in the phrase a great many books, is really a partitive genitive.]
12. A reduced form of the preposition on, formerly common in all the uses of on, but now restricted to certain constructions in which the preposition is more or less disguised, being usually written as one word with the following noun.
13. Of place: On, in, upon, unto, into; the preposition and the following noun being usually written as one word, sometimes with, but commonly without, a hyphen, and regarded as an adverb or a predicate adjective, but best treated as a prepositional phrase. Similarly In such phrases a denotes
14. Of state: On, in, etc.: as, to be alive
15. to be asleep
16. to set afire; to be afloat; to set adrift. In this use now applicable to any verb (but chiefly to monosyllables and dissyllables) taken as a noun: as, to be aglow with excitement; to be a-swim; to be all a-tremble.
17. Of time: On, in, at, by, etc., remaining in some colloquial expressions: as, to stay out a nights (often written o' nights); to go fishing a Sunday; now a days (generally written nowadays). , , Common with adverbs of repetition: as, twice a day
18. Of process: In course of, with a verbal noun in -ing, taken passively: as, the house is a building; “while the ark was a preparing”(1 Pet. iii. 20); while these things were a doing. The prepositional use is clearly seen in the alternative construction with in: as, “Forty and six years was this temple in building,” John ii. 20. In modern use the preposition is omitted, and the verbal noun is treated as a present participle taken passively: as, the house is building. But none of these forms of expression has become thoroughly popular, the popular instinct being shown in the recent development of the desired “progressive passive participle”: as, the house is being built, the work is being done, etc. This construction, though condemned by logicians and purists, is well established in popular speech, and will probably pass into correct literary usage.
19. Of action: In, to, into; with a verbal noun in -ing, taken actively. With be: as, to be a coming; to be a doing; to be a fighting. Now only colloquial or provincial, literary usage omitting the preposition, and treating the verbal noun as a present participle: as, to be coming; to be doing.
20. A reduced form of of, now generally written o', as in man-o'-war, six o'clock, etc.
21. A modern provincial corruption of the pronoun I.
22. An old (and modern provincial) corruption of all genders and both numbers of the third personal pronoun, he, she, it, they. So quotha, that is, quoth he.
23. An old (and modern provincial) corruption of have as an auxiliary verb, unaccented, and formerly also as a principal verb.
24. All.
25. The early form of ah, preserved, archaically, before a leader's or chieftain's name, as a war-cry (but now treated and pronounced as the indefinite article).
26. A Latin preposition, meaning of, off, away from, etc. It occurs in certain phrases: as, a priori, a posteriori, a mensa et thoro, etc.; also in certain personal names of medieval or modern origin: as, Thomas à Kempis, that is, Thomas of Kempen, the school-name given to Thomas Hammerken, born at Kempen near Düsseldorf; Abraham a Sancta Clara, that is, Abraham of St. Clare, the name assumed by Ulrich Megerle. The true name of Thomas a Becket (written also A' Becket. and, in un-English fashion, à Becket, à Becket) was simply Thomas Becket or Beket; the a appears to be a later insertion, though supported by such late Middle English names as Wydo del Beck't, John de Beckote, William atte Beck, etc., that is, of or at the brook
27. A prefix or an initial and generally inseparable particle. It is a relic of various Teutonic and classical particles, as follows:
28. An unaccented inseparable prefix of verbs, and of nouns and adjectives thence derived, originally implying motion away, but in earlier English merely intensive, or, as in modern English, without assignable force, as in abide, abode, arise, awake, ago = agone, etc. The difference between abide, arise, awake, etc., and the simple verbs bide, rise, wake, etc., is chiefly syllabic or rhythmic. In a few verbs this prefix has taken in spelling a Latin semblance, as in accurse, affright, allay, for a-curse, a-fright, a-lay.
29. An apparent prefix, properly a preposition, the same as a, preposition When used before a substantive it forms what is really a prepositional phrase, which is now generally written as one word, with or without a hyphen, and regarded as an adverb or as a predicate adjective: as, to lie abed, to be asleep, to be all a-tremble, etc. With verbal nouns in -ing it forms what is regarded as a present participle, either active, as, they are a-coming (colloq.), or passive, as, the house was a-building. In the latter uses the a is usually, and in all it would be properly, written separately, as a preposition. See a, preposition, where the uses are explained.
30. A prefix, being a reduced form of Anglo-Saxon of, prep., English off, from, as in adown (which see), or of later English of, as in anew, afresh, akin, etc. (which see).
31. A prefix, being a reduced form of Anglo-Saxon of-, an intensive prefix, as in athirst, ahungered (which see).
32. A prefix, being a reduced form of and- (which see), as in along (which see).
33. A prefix, being one of the reduced forms of the Anglo-Saxon prefix ge- (see i-), as in along
34. aware
35. aford, now spelled afford, simulating the Latin prefix af-
36. among
37. etc. The same prefix is otherwise spelled in enough, iwis, yclept, etc.
38. A prefix, being a reduced form of at-, mixed with a- for on-, in afore (which see).
39. A prefix, in ado, originally at do, northern English infinitive, equivalent to English to do. See ado.
40. A quasi-prefix, a mere opening syllable, in the interjections aha, ahoy. In aha, and as well in ahoy, it may be considered as ah.
41. A quasi-prefix, a mere opening syllable, in avast, where a-, however, represents historically Dutch houd in the original Dutch expression houd vast = English hold fast.
42. A prefix, being a reduced form of the Latin prefix ad-. , , In Old French and Middle English regularly a-, and so properly in modern French and English, as in avouch
43. A prefix, being a reduced form (in Latin, and so in English, etc.) of the Latin prefix ad- before sc-, sp-, st-, and gn-, as in ascend, aspire, aspect, astringent, agnate, etc.
44. A prefix, being a reduced form (in Middle English, etc.) of Latin ab-, as in abate (which see). In a few verbs this a- has taken a Latin semblance, as in abs-tain (treated as ab-stain), as-soil. See these words.
45. A prefix, being a reduced form (in Latin, and so in English, etc.) of the Latin prefix ab-, from, as in avert (which see).
46. A prefix, being an altered form of e-, reduced form of Latin ex-, as in amend, abash, etc., aforce, afray (now afforce, affray), etc. (which see).
47. A prefix, being a reduced form of an- for en-, in some words now obsolete or spelled in semblance of the Latin, or restored, as in acloy, acumber, apair, etc., later accloy, accumber, modern encumber, impair, etc.
48. A quasi-prefix, representing original Latin ah, interj., in alas (which see).
49. A prefix of Greek origin, called alpha privative, the same as English un-, meaning not, without, -less, used not only in words taken directly or through Latin from the Greek, as abyss, adamant, acatalectic, etc., but also as a naturalized English prefix in new formations, as achromatic, asexual, etc., especially in scientific terms, English or New Latin, as Apteryx, Asiphonata, etc.
50. A prefix of Greek origin, occurring unfelt in English acolyte, adelphous, etc.
51. A prefix of Greek origin, occurring unfelt in atlas, amaurosis, etc.
52. A prefix of Arabic origin, occurring unfelt in apricot, azimuth, hazard (for *azard), etc., commonly in the full form al-. See al-.
53. A suffix characteristic of feminine nouns and adjectives of Greek or Latin origin or semblance, many of which have been adopted in English without change. ; ; ; . Examples are: Greek (first declension — in Latin spelling), idea, coma, basilica, mania, etc.
54. A suffix, the nominative neuter plural ending of nouns and adjectives of the second and third declensions in Greek or Latin, some of which have been adopted in English without change of ending. ; Examples are: in Greek, phenomena, plural of phenomenon, miasmata, plural of miasma(t-), etc.
55. An unmeaning syllable, used in old ballads and songs to fill out a line.
56. . In music, the A next above middle C has (at French pitch) 435 vibrations per second. In medieval music, the final of the Æolian and hypoæolian modes.
57. In chem., the symbol for argon.
58. Also an abbreviation of ampere and of A-level (which see).

Wiktionary

1. The first letter of the alphabet, called [[a#|a]] and written in the Latin script.
2. The ordinal number first, derived from this letter of the alphabet, called [[a#|a]] and written in the Latin script.
3. n. The name of the Latin script letter [[A#|A]]/[[a#|a]].
4. n. The best grade; superiority.
5. One; any indefinite example of.
6. One certain or particular.
7. The same; one.
8. prep. In, on, at, by.
9. prep. In the process of; in the act of; into; to. (Used with verbal substantives in -ing which begin with a consonant.)
10. prep. Of.
11. prep. To, each, per.
12. v. Have. (Now often attached to preceding auxiliary verb.)
13. pro. He; she; it; they.
14. interj. A meaningless syllable; ah.
15. abbr. accepted
16. abbr. ante; before
17. abbr. active
18. abbr. adjective
19. abbr. An are, a unit of area of which 100 comprise a hectare.
20. The first letter of the alphabet, called [[a#|a]] and written in the Latin script.
21. The ordinal number first, derived from this letter of the alphabet, called [[a#|a]] and written in the Latin script.
22. n. The name of the Latin script letter [[A#|A]]/[[a#|a]].
23. n. The best grade; superiority.
24. One; any indefinite example of.
25. One certain or particular.
26. The same; one.
27. prep. In, on, at, by.
28. prep. In the process of; in the act of; into; to. (Used with verbal substantives in -ing which begin with a consonant.)
29. prep. Of.
30. prep. To, each, per.
31. v. Have. (Now often attached to preceding auxiliary verb.)
32. pro. He; she; it; they.
33. interj. A meaningless syllable; ah.
34. abbr. accepted
35. abbr. ante; before
36. abbr. active
37. abbr. adjective
38. abbr. An are, a unit of area of which 100 comprise a hectare.

GNU Webster's 1913

1. The first letter of the English and of many other alphabets. The capital A of the alphabets of Middle and Western Europe, as also the small letter (a), besides the forms in Italic, black letter, etc., are all descended from the old Latin A, which was borrowed from the Greek Alpha, of the same form; and this was made from the first letter (�) of the Phœnician alphabet, the equivalent of the Hebrew Aleph, and itself from the Egyptian origin. The Aleph was a consonant letter, with a guttural breath sound that was not an element of Greek articulation; and the Greeks took it to represent their vowel Alpha with the ä sound, the Phœnician alphabet having no vowel symbols.
2. The name of the sixth tone in the model major scale (that in C), or the first tone of the minor scale, which is named after it the scale in A minor. The second string of the violin is tuned to the A in the treble staff. -- A sharp (A♯) is the name of a musical tone intermediate between A and B. -- A flat (A♭) is the name of a tone intermediate between A and G.
3. An adjective, commonly called the indefinite article, and signifying one or any, but less emphatically.
4. In each; to or for each
5. prep. In; on; at; by.
6. prep. In process of; in the act of; into; to; -- used with verbal substantives in -ing which begin with a consonant. This is a shortened form of the preposition an (which was used before the vowel sound); as in a hunting, a building, a begging.
7. Of.
8. A barbarous corruption of have, of he, and sometimes of it and of they.
9. An expletive, void of sense, to fill up the meter.

WordNet 3.0

1. n. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
2. n. the basic unit of electric current adopted under the Systeme International d'Unites
3. n. the blood group whose red cells carry the A antigen
4. n. one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
5. n. any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
6. n. a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
7. n. the 1st letter of the Roman alphabet